Method of manufacturing strain sensor

ABSTRACT

A strain sensor comprising a metal substrate, a first electrode provided on the metal substrate, a glass layer formed on the first electrode, and a second electrode and a strain detecting resistor provided on the glass layer. In the strain sensor, the insulation resistance between the metal substrate and the second electrode has been raised, and the reliability is high. It can be implemented at low cost.

This is a Division of application Ser. No. 10/451,668 filed Jun. 25,2003, now U.S. Pat. No. 7,010,986, which in turn is a national stageentry of International Application No. PCT/JP 02/10259 filed Oct. 2,2002 designating the U.S., which claims the benefit of JapaneseApplication No. 2001-306132 filed Oct. 2, 2001.

TECHNICAL FIELD

The present invention relates to a strain sensor which detects anexternal force generated by the weight of a human body, weight of avehicle, etc., through detection of a strain caused in a straindetecting resistor.

BACKGROUND ART

A conventional strain sensor disclosed in Japanese Patent Laid-OpenApplication No. 2000-180255 is structured of one glass layer and oneovercoat glass layer. In the following, a conventional strain sensor isdescribed referring to drawings. FIG. 7 shows plan view of aconventional strain sensor.

Metal substrate 1 is provided at one end with first fixing hole 20,second fixing hole 21 at the other end, and detection hole 22 in theapproximate middle part. On the upper surface of metal substrate 1,glass layer 2 is formed, and four strain detecting resistors 6 areprovided thereon. Strain detecting resistors 6 are electricallyconnected by wiring 11 and second electrode 5 to form a bridge circuit.Strain detecting resistors 6 and second electrode 5 are protected byovercoat glass layers 7 a, 7 b.

FIG. 8 shows cross sectional view of a conventional strain sensor,sectioned along the line A–A′ of FIG. 7. Since glass layers 2 a, 2 b and2 c are made of same lead borosilicate system glass material, theselayers are integrated into a single layer after being sintered. So,individual glass layers can not be distinguished severally in the layer.

In FIG. 8, these glass layers 2 a, 2 b and 2 c are illustrated severallyby providing a broken line between the layers for the sake of easyunderstanding. Round voids 9 are scattered at random within the glasslayers after sintering.

Reason why the glass layer in FIG. 8 is illustrated in three layers isthat Japanese Patent Laid-Open Application No. H09-243472 teachesformation of a multi-layered insulation layer by printing a 20 μm thickglass paste of lead boro-silicate system glass material and sintering itfor three times (printing-sintering is repeated for three times),instead of using enamel or crystalline glass.

A method of assembling a conventional strain sensor is described in thefollowing with reference to FIG. 7.

On the upper surface of already-prepared metal substrate 1, a glasspaste is screen-printed and sintered at an approximate temperature of850° C. for forming glass layer 2 on the upper surface of metalsubstrate 1. On the upper surface of glass layer 2, a conductive pasteof Ag and Pt is screen-printed and sintered at an approximatetemperature of 850° C. for forming wiring 11 and second electrode 5 onthe upper surface of glass layer 2. And then, a Ru system resistancepaste is printed covering part of glass layer 2 and second electrode 5,and sintered at an approximate temperature of 850° C. for forming straindetecting resistor 6. Finally, a glass paste is screen-printed coveringglass layer 2, wiring 11, strain detecting resistor 6, and sintered forforming overcoat glass layers 7 a, 7 b.

If a window is provided in advance in the pattern of overcoat glasslayer, a chip component or semiconductor device can be mounted andconnected with wiring 11 which is exposed through the window. Operationof the above-configured conventional strain sensor is described below.Metal substrate 1 is fixed, at first fixing hole 20 and second fixinghole 21, on a fixed member (not shown) by means of bolt (not shown) andnut (not shown), and then a detection member (not shown) is fixed todetection hole 22. When an external force F is given from the above onthe detection member (not shown), a deformation is caused on metalsubstrate 1.

As a result, strain detecting resistors 6 disposed on the upper surfaceof metal substrate 1 receive a compressive stress or a tensile stress,and the resistance value in each of respective strain detectingresistors 6 changes. Strain detecting resistors 6 are connected bywiring 11 to form a bridge circuit, and an external force F exerted ondetection member (not shown) is detected in the form of differentialvoltage detected at the bridge. FIG. 4A shows relationship between thenumber of glass layers and the insulation resistance in conventionalstrain sensor as shown in FIG. 7, FIG. 8. FIG. 4A shows that theinsulation resistance is in the level of ninth to eleventh power of 10,when the number of glass layers is as many as 3–4 layers. However, whenthe number of glass layers decreased to 2 layers, the insulationresistance decreased to the level of sixth to tenth power of 10. Whenthe glass layer count decreased to 1, the insulation resistancedecreased further down. According to result of measurement conducted on1-layer glass layers, the insulation resistance was mostly lower than1Ω, or a state of short circuit; only a limited number of samples showedseveral hundreds KΩ. In the graph of FIG. 4A, the insulation resistanceof 1-layer glass is shown to have the fifth power of 10, for the sake ofsimplification. As described in the above, the insulation resistancedecreases sharply when the glass layer counts go lower (or, the glasslayer thickness goes thinner), among those having conventionalstructure. The insulation resistance value also disperses wide, whichgenerates a problem in the products reliability.

In practice, a strain sensor needs an insulation resistance that is inthe ninth power of 10, or higher; which means that in the conventionalstructure three or more glass layers are needed.

This leads to a higher cost of finished products.

DISCLOSURE OF INVENTION

The present invention offers a strain sensor which comprises a metalsubstrate, a first electrode formed on said metal substrate, a glasslayer formed on said first electrode, a second electrode formed on saidglass layer and a strain detecting resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view used to describe a strain sensor inaccordance with the present invention.

FIG. 2A–FIG. 2D are drawings used to describe a method of manufacturinga conventional strain sensor.

FIG. 3A–FIG. 3D are drawings used to describe a method of manufacturinga strain sensor in accordance with the present invention.

FIG. 4A, FIG. 4B are graphs used to describe relationship between thenumber of glass layers and the insulation resistance.

FIG. 5A, FIG. 5B are drawings used describe the voltage between themetal substrate and the second electrode during sintering process.

FIG. 6 shows a cross sectional view used to describe the pattern offirst electrode.

FIG. 7 shows plan view of a conventional strain sensor.

FIG. 8 shows cross sectional view of a conventional strain sensor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 shows the structure of a strain sensor in accordance with anexemplary embodiment of the present invention. The drawing is intendedto provide the concept of the invention, so the illustration does notgive precise dimensions. On the upper surface of metal substrate 1,first electrode 12 is formed, and second electrode 5, strain detectingresistor 6 connected with second electrode 5 and wiring 11 are providedthereon via glass layer 2.

Wiring 11 is connected to second electrode 5 with a certain specificpattern. Overcoat glass layer 7 is further provided thereon.

Round voids 9 are voids generated at sintering process. A key point ofdifference of FIG. 1 as compared to FIG. 7, or the conventionalstructure, is whether there is first electrode 12 disposed between metalsubstrate 1 and glass layer 2, or not.

In the present invention, first electrode 12 is inserted between secondelectrode 5, wiring 11 and metal substrate 1; this contributes to keepthe insulation resistance between second electrode 5, wiring 11 andmetal substrate 1 high.

As a result, the short-circuit rejects can be reduced significantly.

Next, a method of manufacturing is described referring to FIG. 2A–FIG.4B.

FIG. 2A–FIG. 2D are used to describe a method of manufacturingconventional strain sensors. FIG. 2A shows metal substrate 1. Metalsubstrate 1 may be provided optionally in various shapes depending onneeds.

Next, as shown in FIG. 2B, glass layer 2 is formed on the upper surfaceof metal substrate 1. The glass layer is formed by screen-printing aglass paste normally available in the market and sintering it at 850° C.using a belt furnace. Furthermore, glass layer 3 is formed thereon, asshown in FIG. 2C. Providing glass layer for more than one layer, viz. inthe multi-layered way, is effective to prevent generation of pin-holesor the like trouble which might arise during the printing process. Next,as shown in FIG. 2D, second electrode 5 is provided on the glass layerof multi-layered structure.

In the actual manufacturing process of strain sensors, second electrode5 has a complicated pattern shape, and strain detecting resistors andprotection layer are provided thereon before completing finished strainsensors.

FIG. 2D shows how the insulation resistance is measured between secondelectrode 5 and metal substrate 1 with insulation resistance meter 13.Drawings in FIG. 3 are used to describe a method of manufacturing strainsensor in accordance with a first exemplary embodiment of the presentinvention. FIG. 3A shows a state in which first electrode 12 has beenformed on metal substrate 1. Metal substrate 1 is made of a 2 mm thickstainless steel sheet. First electrode 12 is formed by screen-printingan electrode ink containing silver as the main ingredient in a certainspecific pattern on the metal substrate, and sintering it in a beltfurnace at 850° C. Next, as shown in FIG. 3B, glass layers 2 and 3 areprovided on first electrode 12.

As shown in FIG. 3C, second electrode 5 is formed on glass layer 3.Using the samples thus provided, the insulation resistance betweensecond electrode 5 and metal substrate 1, or between second electrode 5and first electrode 12, is measured with insulation resistance meter 13,as illustrated in FIG. 3D. FIG. 4A and FIG. 4B show relationship betweenthe number of glass layers and the insulation resistance among thesamples which were described referring to FIG. 2 or FIG. 3. The X axisrepresents the number of glass layers, while Y axis the insulationresistance, unit used there is Ω.

A plurality of measured data 14 has been approximated into extrapolationline 15. FIG. 4A shows the insulation resistance between secondelectrode 5 and metal substrate 1 in the conventional structure of FIG.2A–FIG. 2D (corresponding to FIG. 7). FIG. 4A shows that the insulationresistance is high when glass layer counts are as many as 3 to 4 layers,while it steeply decreases along with the decreasing layer counts. FIG.4A also teaches that the insulation resistance values disperse widely,despite an effort to reduce the influence of pin-holes contained withinthe glass layers by providing the glass layer in a multi-layeredstructure. FIG. 4B shows the insulation resistance between secondelectrode 5 and metal substrate 1 in a structure in accordance with thepresent invention, as shown in FIG. 3A to FIG. 3D (corresponding to FIG.1). From FIG. 4B, it is seen that the insulation resistance stays withina range of 10 in the power of ten to eleven, whole through the glasslayer counts from 1-layer to 4-layers. Although the insulationresistance decreases slightly along with the decreasing layer counts,from 3-layers, 2-layers to 1-layer, even 1-layered glass layer securesthe insulation resistance value to be higher than the tenth power of 10.

Thus in the case of FIG. 4B, even a single-layered glass layer canprovide enough insulation resistance. The glass layer may of course beprovided in a multi-layered structure, which will be advantageous insuppressing the pin holes due to dust, etc.

Experiments were conducted with the samples that correspond to FIG. 2Athrough FIG. 4B, using different materials for first electrode 12 andsecond electrode 5 in the samples. According to the experimentalresults, gold (Au) electrode does not exhibit the difference as shownbetween FIG. 4A and FIG. 4B. It has become known from the experimentsthat a sintered electrode containing Ag (Ag—Pd, Ag—Pt, Ag—Pd—Pt, etc.)used for second electrode 5 is prone to cause the phenomenon of steepdecrease in the insulation resistance between metal substrate 1 andsecond electrode 5, occurred as a result of decreased number of glasslayer counts as shown in FIG. 4A. Samples whose electrode paste havingno glass component additive likewise exhibited the phenomenon of FIG.4A. In other experiments in which different kinds of glass (material,softening point, etc.) were used for the glass layer, the steep decreaseof insulation resistance was not observed in a region before the glasssoftening point. The phenomenon readily appeared after the glassexceeded the softening point. Meanwhile, when metal substrate 1 isprovided on the surface with first electrode 12 in accordance with thepresent invention, no such decrease in the insulation resistance wasobserved, regardless of difference in the kind of glass materials andthe softening point.

The sample of FIG. 3A, having no first electrode 12 (the structure ofFIG. 7), and the sample of FIG. 3B, having first electrode 12 (thestructure of FIG. 1), were scrutinized with SEM in their cross sections.Both of the samples had round void 9, which was described referring toFIG. 7. Although first electrode 12 can not eliminate the voidscontained within glass layer, it substantially improves the reliabilityof such finished products where the number of glass layers has beenreduced (glass layer has been thinned).

Thus, a superior quality is ensured with the products, and the number ofprocess steps can be reduced to a cost saving. FIG. 5A and FIG. 5B areillustrations used to describe voltage generated between metal substrateand second electrode during sintering. FIG. 5A shows results of thevoltage measurement conducted on the samples set in a single furnace, byreading the voltages generated during the sintering procedure.

FIG. 5B shows how the voltage is measured during sintering with thetrial sample. As FIG. 5B illustrates, metal substrate 1, secondelectrode 5, etc. are connected with platinum wire at one end, and thesample in this state is put in a single furnace and the furnacetemperature is ramped up (single furnace is not shown). Voltagesgenerated with the sample are measured with a voltmeter connected to theother end of the platinum wire coming out of the single furnace.

In FIG. 5A, curve 16 represents the conventional sample, while curve 17the present invention. The X axis indicates temperature of athermocouple disposed in the single furnace, while Y axis indicatesvoltage read by the voltmeter. Curve 17 shows the voltages with thesample having first electrode (the structure of FIG. 3). It is seen thatthere is hardly any voltage generated between metal substrate 1 andsecond wiring 5. On the other hand, curve 16 shows the voltages with thesample having no first electrode (the structure of FIG. 2). It is seenthat there are voltages generated between metal substrate 1 and secondwiring 5. Some minor voltage starts to emerge at the vicinity of 600°C., it reaches approximately 1V at the vicinity of 850° C. When it iskept at 850° C. for 30 min., the voltage increases to approximately 1.5V. Even after the temperature is lowered from 850° C., the voltage stillstays at high side. This seems to have been caused by the difference inthermal capacitance between the thermocouple used for temperaturemeasurement and the metal substrate; or difference in temperaturebetween the actual temperature inside the single furnace and that ofsample itself.

Respective samples were likewise heated and measured in the voltageusing heating means other than the single furnace. This experimentrendered the same results as described in the above.

From the results made available in FIG. 4A through FIG. 5B, thefollowing inference may be drawn: The phenomenon of steeply decreasinginsulation resistance, as shown in FIG. 4A, is particular to the casewhen a sintered electrode material containing at least Ag is used forsecond electrode 5. In the course of forming second electrode 5 (850° C.sintering), the glass (850° C. sintering) melts again. Then, the glassfunctions as a kind of solid electrolytic material, and generates acertain difference in the potential between metal substrate 1 and secondelectrode 5. This further works as a kind of battery, and invites asudden decrease in the insulation resistance. First electrode 12 formedon metal substrate 1 effectively prevents generation of such voltages asillustrated in FIG. 5A.

It seems that, as a result of above scenario, even the sample havingless number of glass layer counts (or, when thickness of glass layer isthin) can provide such a high insulation resistance as shown in FIG. 4B.When the glass layer is provided in the multi-layered structure, or thelayer thickness of glass layer is made to be thicker than a certainspecific value, occurrence of the pin hole due to dust, etc. can besuppressed and the strain sensors can be manufactured with a highproduction yield rate.

Embodiment 2

Now, description is made with reference to FIG. 6 on the patterned shapeof first electrode 12.

First electrode 12 is provided in the form of electrode pattern 18,which is disposed between metal substrate 1 and glass layer 2. For thepurpose of preventing the battery effect generated between secondelectrode 5 and metal substrate 1, first electrode 18 does notnecessarily need to be an entire plate. It may come either in such aregular pattern as mesh, checkers, polka dots, zigzags, or in a randompattern. First electrode provided in a patterned shape, instead of anentire plate, contributes to save the electrode material used for firstelectrode pattern, which leads to a cost reduction of finished products.

Embodiment 3

Now in embodiment 3, description is made on the composition of firstelectrode.

(Table 1) shows experimental results conducted on the composition offirst electrode 12. The glass composition of SiO₂ alone does not providesufficient adhesive strength.

A certain necessary adhesive strength is available by adding componentsof PbO, CaO, Al₂O₃, etc. for about 1 weight % to SiO₂. When Bi₂O₃ wasused, preferred quantity was not less than 1 weight % not more than 10weight %. If glass component is in excess of 30 weight %, its electricalresistance was too high, although enough adhesive strength was provided.This is not favorable in view of possible deterioration in theelectrical characteristics of finished strain sensors.

TABLE 1 Component in first Adhesive Electrical Glass Compositionelectrode (weight %) strength resistance SiO2 (0–10) X (peeled off) ◯SiO2 (0.5–5) X (peeled off) ◯ Si02 + Al203 (0.5–5) + (0.1–5) ◯ ⊚ Si02 +Pb0 (0.5–5) + (0.1–5) ◯ ⊚ (0.5–5) + (0.1–5) Si02 + Ca0 (0.5–5) + (1–10)◯ ⊚ Si02 + Bi203 ◯ ⊚ Si02 + Al203 + (0.5–5) + 1 + (1–10) ⊚ ⊚ Bi203Si02 + Pb0 + Al203 (0.5–5) + 1 + (0.1–5) ⊚ ⊚ Si02 + Bi203 + Pb0(0.5–5) + (1–10) + 1 ⊚ ⊚ Si02 + Pb0 + Ca0 (0.5–5) + 1 + (0.1–5) ⊚ ⊚Si02 + Al203 + (0.5–5) + (0.1–5) + ⊚ ⊚ Pb0 + Bi203 + (0.1–5) + (1–10) +Ca0 (0.1–5), Total 25 weight % Si02 + Al203 + Total 30 weight % ⊚ XPb0 + Bi203 + Ca0 (◯ Good, ⊚ Superior, X Rejected), Notes (1–10): notless than 1 weight %, not more than 10 weight %. (1–2): not less than 1weight %, not more than 2 weight %.

Embodiment 4

Now in embodiment 4, description is made on the composition of secondelectrode.

(Table 2) shows results of study conducted on the composition of secondelectrode 5.

The (Al₂O₃+SiO₂ alone does not provide enough adhesive strength.However, a certain necessary adhesive strength is available by addingcomponents of CaO, NiO, CuO, PbO, etc. for about 0.1–1 weight %.

When Bi₂O₃ is used as the additive, preferred quantity is not less than1 weight % not more than 10 weight %.

If glass component is in excess of 30 weight %, its electricalresistance goes too high, although enough adhesive strength is provided.This is not favorable in view of possible deterioration in theelectrical characteristics of finished strain sensors. Addition of Pdcomponent to second electrode 5 is effective to prevent a solderleaching. Second electrode 5 and wiring 11 may be put into a commonmember; by so doing, the number of printing process steps can be reducedto a cost reduction. Second electrode 5 can be used as wiring 11, onwhich other chip components or semiconductor devices may be mounted withsolder.

When adding Pd component to second electrode, preferred adding quantityis not less than 5 weight % not more than 15 weight %. If it is lessthan 5 weight %, the effect of preventing a solder leaching isinsufficient. If it is in excess of 15 weight %, the wiring resistancegoes high and the cost of electrode also goes up. So, this is notpreferred.

TABLE 2 Component in second Adhesive Electrical Glass Compositionelectrode (weight %) strength resistance (Al2O3 + SiO2) (0.1–5) +(0.5–5) X (peeled off) ⊚ (Al2O3 + SiO2) + CaO (0.1–5) + (0.5–5) +(0.1–5) ◯ ⊚ (Al2O3 + SiO2) + NiO (0.1–5) + (0.5–5) + (0.1–5) ◯ ⊚(Al2O3 + SiO2) + CuO (0.1–5) + (0.5–5) + (0.1–5) ◯ ⊚ (Al2O3 + SiO2) +Bi2O3 (0.1–5) + (0.5–5) + (1–10) ◯ ⊚ (Al2O3 + SiO2) + (0.1–5) +(0.5–5) + (0.1–5) + ⊚ ⊚ CaO + NiO + Bi2O3 (0.1–5) + (1–10) (Al2O3 +SiO2) + NiO + (0.1–5) + (0.5–5) + (0.1–5) + ⊚ ⊚ CuO + Bi2O3 (0.1–5) +(1–10) (Al2O3 + SiO2) + CaO + (0.1–5) + (0.5–5) + (0.1–5) + ⊚ ⊚ NiO +CuO (0.1–5) + (0.1–5) (Al203 + Si02) + Ca0 + (0.1–5) + (0.5–5) +(0.1–5) + ⊚ ⊚ Ni0 + Cu0 + Bi203 (0.1–5) + (0.1–5) + (1–10) Total 25weight % (Al203 + Si02) + Ca0 + Co203 + Total 30 weight % ⊚ X Ni0 +Cu0 + Bi203 (◯ Good, ⊚ Superior, X Rejected) Notes (0.1–5): not lessthan 0.1 weight %, not more than 5 weight %. (0.5–5): not less than 0.5weight %, not more than 5 weight %.

Embodiment 5

In embodiment 5, description is made on the Ag quantity contained inelectrode. As to the materials for first electrode and second electrode,a certain resistance value needed for an electrode layer may be providedby having them to include Ag component for not less than 40 weight %(preferably not less than 60 weight %). Further addition of Pd, Pt, etc,enhances the reliability required for a wiring member. If the Agcomponent in first and second electrodes is not more than 40 weight %,resistance value of the electrode layers becomes too high and theproduct characteristics deteriorate. If glass component is in excess of30 weight %, its electrical resistance goes too high; this is notfavorable in view of possible deterioration in the electricalcharacteristics of finished strain sensors. As described in the above,even a single-layered glass layer can maintain the insulation resistancehigh, and can reduce substantially the rate of short-circuit rejects.

If it is provided by laminating a plurality of glass layers, the rate ofrejects due to pin hole, etc, can be significantly reduced to animproved production yield rate. The thermal expansion coefficient ofglass layer can be harmonized with that of metal substrate, when theglass layer is formed with at least SiO₂, Al₂O₃, BaO as the mainingredient. Thickness of the glass layer should preferably be not lessthan 5 μm not more than 500 μm. If the thickness of glass layer is notmore than 5 μm, the insulation resistance is sometimes lowered due tomicro-holes, etc. generated in the glass layer, despite the formation offirst electrode. On the other hand, a glass layer thicker than 500 μminvites an increased product cost.

First electrode, glass layer, second electrode, etc. should preferablybe sintered in an oxidizing atmosphere at a temperature not lower than600° C. not higher than 1000° C. If they are sintered at a temperaturenot higher than 600° C., sintering of electrode and glass layer mightremain insufficient, which would lead to the insufficiency of physicalstrength and characteristics.

On the other hand, when the sintering temperature is higher than 1000°C., the electrode, the glass layer and even the metal substrate itselfare required to be formed of materials of special kinds, which meansexpensive, and it may require an expensive sintering furnace. So, it isnot a favorable choice. It is preferred that these items are sintered inan oxidizing atmosphere. If they are sintered in a reductive atmosphere,part of materials constituting glass layer (PbO, for example) issometimes reduced, which would cause a lowered insulation resistance.The strain detecting resistor should preferably be sintered at atemperature not lower than 500° C. not higher than 900° C. If it issintered at a temperature not higher than 500° C., or at a temperaturenot lower than 900° C., a certain specific strain detectingcharacteristic might not be provided. When an overcoat glass layer isused for the protection layer, an oxidizing atmosphere not lower than400° C. not higher than 800° C. is preferred.

A temperature not higher than 400° C. might fail to provide a certainspecific reliability.

If sintered at a temperature not lower than 800° C., the straindetecting characteristic might deteriorate. When sintered in a reductiveatmosphere, part of glass components contained in the overcoat glasslayer (PbO, for example) is sometimes reduced, which would fail toprovide a certain specific reliability. Thickness of first electrodeshould preferably be not less than 0.1 μm not more than 100 μm. If it isthicker than 100 μm, the electrode might peel off the metal substrate byan internal stress generated during sintering. Furthermore, it creates asignificant stepped level difference in the glass layer caused by thethick first electrode. This would bring about a problem in the latermanufacturing steps. At the same time, this is a factor of costincrease. If thickness of first electrode is not more than 0.1 μm, itmight fail to provide the glass layer with the effect for preventing thedecreasing insulation resistance.

In the descriptions of the present invention, the adding quantity ofrespective components is expressed in terms of weight %, while oxidesare expressed in terms of weight % as oxide. Proportion of theconstituent elements can be made available by applying the XMA method(X-ray Micro-Analyzer) to the cross sectional surface, etc. of afinished product, or through the generally-available methods. In thiscase, the element is calculated in the oxide (for example, Al is handledas Al₂O₃).

As described in the above, when a structure consisting of metalsubstrate-glass-wiring is sintered, sometimes a certain voltage isgenerated between the metal substrate and the wiring, depending on kindof the glass and electrode materials. The voltage sometimes bring abouta lowered insulation resistance between the metal substrate and thewiring.

The generation of voltage between metal substrate and wiring can beavoided by providing a silver wiring direct on the metal substrate.Thus, the drop of insulation resistance between metal substrate andwiring can be avoided. Furthermore, since electrical resistance of metalsubstrate itself can be lowered by first electrode formed on the metalsubstrate, it will provide designers of electrical circuits for strainsensors with more designing freedom and more room for the optimization.

Therefore, it is advantageous in improving the electricalcharacteristics of a strain sensor to provide also to such a glass whichhardly generate a voltage during sintering with first electrode. As tomaterial for the metal substrate, a ferrite system stainless steelcontaining Cr, Al₂O₃ is preferred in view of the heat-resistingproperty. A stainless steel substrate containing, for example, Cr fornot less than 5 weight % not more than 20 weight % and Al₂O₃ for notless than 2 weight % not more than 10 weight %, will provide asufficient physical strength needed for a strain sensor, as well asenough heat-resisting property withstanding a temperature up to thevicinity of 900° C. Furthermore, by adding Cr and Al₂O₃, which beingcommon constituent elements with the metal substrate, also to the firstelectrode side, the mutual connection can be made more stable.

INDUSTRIAL APPLICABILITY

Reliable strain sensors can be offered at low cost, by providing metalsubstrate with first electrode in accordance with the present invention.Which electrode enhances the insulation resistance between metalsubstrate and second electrode.

1. A method of manufacturing strain sensor comprising the steps of; providing a first electrode on a metal substrate, forming a glass layer for at least one layer on said first electrode, providing a second electrode and a wiring on said glass layer, providing a strain detecting resistor electrically connected with said second electrode, and forming an overcoat glass layer which covers at least one of said second electrode and said strain detecting resistor.
 2. The method of manufacturing strain sensor recited in claim 1, wherein said first electrode, said glass layer and said second electrode are all sintered in an oxidizing atmosphere at a temperature not lower than 600° C. not higher than 900° C.
 3. The method of manufacturing strain sensor recited in claim 1, wherein said overcoat glass layer is sintered in an oxidizing atmosphere at a temperature not lower than 400° C. not higher than 800° C. 